1. Field of the Invention
The present invention relates to a fiber-to-fiber optical coupler and, more particularly, to a non-tapered, butt-coupled, fused-fiber coupler and method of forming the same that eliminates the problems associated with prior arrangements which utilized a tapered fiber and/or precise fiber alignment processes during the manufacture of the coupler.
2. Description of the Prior Art
As optical fiber communications becomes more prevalent, methods of improving the performance of optical fiber systems become necessary. Many of these systems are now used in what can be described as a local area network (LAN) wherein many different users are interconnected, and may also be connected to a central control unit. These interconnections thus demand an optical fiber coupler which is capable of coupling a single fiber to a group of, for example, two or three, other fibers. Several methods of fabricating optical fiber couplers are well known in the prior art. Of the various methods, the fused-fiber approach seems to have received more general acceptance, primarily because such couplers do not require the use of any auxiliary optics.
One early type of fused-fiber device for joining multimode optical fibers is disclosed in U.S. Pat. No. 4,083,625 issued to M. C. Hudson on Apr. 11, 1978. In the Hudson arrangement, a pair of optical fibers are permanently coupled with a third optical fiber using a fusion technique where the end portions of the pair of fibers are fused together in a side-by-side relationship and the endface of the third fiber is fused to the endfaces of the pair of fibers. Inherent in this arrangement is the utilization of tapering technique, an accurate coplanar alignment of the output fiber pairs, and subsequently a precise mating between the input and output fibers. This procedure also requires accurate fiber cleaving since the diameter of the fibers must be controlled.
An alternative arrangement is disclosed in U.S. Pat. No. 4,135,779 issued to M. C. Hudson on Jan. 23, 1979. Here, a variable ratio coupling device is disclosed, similar to the above-described device, with the addition of a means to provide lateral movement between the endfaces of the pair of fibers and that of the third fiber to vary the amount of light coupled between the pair of fibers and the third fiber. This arrangement, while useful with a 1-to-2 coupler, becomes difficult to accomplish with any greater number of fibers, for example a 1-to-3 coupler, where the latter type of coupler is commonly used.
The fused coupler which has has become the most prevalent is formed using a technique known as a biconical taper. This technique is described in U.S. Pat. No. 4,291,940 issued to B. S. Kawasaki et al on Sept. 29, 1981. As disclosed, this technique forms a coupler comprising a first and a second multimode fiber, each of the fibers having a biconical taper section, the biconical taper sections of the fibers being twisted around one another and fused together along a predetermined length. Couplers made in this manner have several drawbacks. In particular, the taper region is drawn to a small diameter and is suspended in air since direct contact with a potting compound can increase loss and/or affect the splitting ratio. Such an air suspension suggests potential mechanical reliability problems and further complicates hermetic and high pressure package design. Additionally, the biconical taper arrangement has not been found to be a successful way of forming 1.times.N couplers for N&gt;2, for nonuniform intensity distributions among the output fibers usually results.
A problem remaining in the prior art therefore, is to provide a method of forming optical couplers which can accommodate, a 1-to-N coupling need, does not require the use of a tapered section, is rugged, and may be directly potted.